The first question leads to an examination of bridge design. Trusses with minimal girth, such as Pratt, Howe or Warren trusses) seem to top out at about 100 feet. The most economical way to span long distances, however, lies in the introduction of tension elements, such as is seen in a suspension or cable-stayed bridge. The most extreme case of a tension maximizing (compression minimizing) design would be the cable/tower arrangements that support ski-lift gondolas. Some have proposed such means as an urban transit system, and they certainly would have something there were it not for the need for multiple origins and destinations.
Many of you may have seen the late Hans Kylberg’s dramatic use of cable-stayed PRT track in the Bubbles and Beams video or in this illustration. Below is a picture of a curved bridge section supported by cables.
The use of cables does present a question of aesthetics. Whereas I think most people would find both types of cable supported bridges generally attractive, there is a question of too much of a good thing. Instead of a minimalist design, there would be support columns and cables everywhere. What looks good crossing a river might not look so good close-up on your street. I have given some thought to reducing the tower height and I will post my ideas for that in the near future.
Then there is the question of track height. One obvious way to mollify the NIMFYs (Not I My Front Yard) is to have the system so elevated that it is unobtrusive. An advantage to the system I have been advocating is that it can easily and steeply climb or descend to any desired level. There is the matter of emergency evacuation, but I think that is manageable. So how high is too high? Should this be a system that can whisk you along above the trees? That would be my preference, but everyone has his own sensibilities. What are yours?